XSS in Gapgag55 Auto Thumbnails

CVE-2026-8899

The Auto Thumbnail plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the 'thumbnails' shortcode in all versions up to, and including, 1.0. This is due to insufficient input sanitization and output escaping on the shortcode's 'width' and 'height' attributes in the athn_thumbnails() function, which are concatenated directly into an HTML <img> tag. This makes it possible for authenticated attackers, with contributor-level access and above, to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page.

Vulnerability class: XSS (Cross-Site Scripting)

Published · last modified .

CVSS v3 metric

CVSS v3 base score 6.4 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:N.

EPSS exploit prediction

EPSS: 0.003 (23.0th percentile), scored .

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 23rd percentile — 23.0% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS trend (30 days)EPSS over the last 30 days for CVE-2026-8899: held from 0.002 to 0.003.

EPSS over last 30 days · oldest: 0.002 · newest: 0.003 · change: +0.001

Affected products

Weakness classification (CWE)

References

Frequently asked questions

What is CVE-2026-8899?
CVE-2026-8899 is a medium-severity vulnerability in Gapgag55 Auto Thumbnails, classified under Cross-site Scripting. CVSS score: 6.4/10. Published 2026-05-27.
How severe is CVE-2026-8899?
Medium severity. CVSS v3 base score is 6.4 out of 10.